Piezoelectric ceramics are used for various purposes as an actuator for ultrasonic vibrators, ultrasonic motors, precise positioning elements, piezoelectric transformers or the like because when a voltage is applied, they undergo elastic deformation; and also as a sensor of an acceleration sensor, a piezoelectric gyroscope for car navigation system, a sonar, an ultrasonic diagnostic element or the like because when deformation is conversely given, they generate a voltage. Recently, a tendency toward intelligence of various machines or systems has become strong, and for that reason, in particular, importance of the actuator is being enhanced. The mainstream of piezoelectric ceramics which are currently used for many purposes is one containing, as a main component, lead titanate zirconate (PZT) and assuming a perovskite structure (ABO3).
Piezoelectric properties of this PZT ceramic are brought through a combination of lead zirconate (PbZrO3) having a rhombohedral structure, which is an antiferroelectric substance, and lead titanate (PbTiO3) having a tetragonal structure, which is a ferroelectric substance, and are the highest in a composition neighboring to a morphotropic phase boundary (MPB) between rhombohedral and tetragonal crystals (neighboring to PbZO3/PbTiO3=52/48). For that reason, many PZT based piezoelectric ceramics are used upon being prepared in a composition neighboring to MPB.
On the other hand, recently, there is a trend for reducing the amount of lead from various materials from the standpoint of a problem of global environmental pollution, and the piezoelectric ceramics are not exceptional.
In fact, almost all of piezoelectric ceramics which are currently used for many purposes and which are represented by PZT ceramics contain a large amount of lead. In particular, PZT contains a large amount of lead, and thus, in recent years, adverse influences against the global environment, such as elution of lead due to acid rain or the like, become problematic. In view of such circumstances, the development of lead-free based piezoelectric ceramic materials having characteristics comparable to PZT is desirable.
A piezoelectric porcelain composition represented by a general formula: (Na,K)NbO3 has a high cubic-tetragonal phase transition temperature (Curie temperature Tc), and thus, in recent years, it is watched as a candidacy substance of the lead-free piezoelectric ceramic as a replacement of PZT (see Patent Documents 1 to 8 and Non-Patent Documents 1 to 9).
Also, (Na,K)NbO3 has a perovskite structure, and when the temperature is changed from a high temperature to a low temperature, it causes sequential cubic-tetragonal-orthorhombic-rhombohedral phase transition.
From Non-Patents 1 to 9, a tetragonal-orthorhombic phase transition temperature (Tc2) can be shifted to a low temperature side by introducing M1M2O3 (wherein M1 represents an element such as Ba, Sr, Ca, Pb, Li or the like; and M2 represents an element such as Ti, Ta, Sb, Nb or the like) into (Na,K)NbO3.
In that case, as Tc2 becomes close to room temperature, the piezoelectric characteristic is improved and becomes maximum in the vicinity of an introduction amount of M1M2O3 of from 3 to 6 mol %. It has been the mainstream for the conventional development of a niobium based lead-free piezoelectric ceramic to enhance the piezoelectric characteristic by selection of the elements and selection of the composition while utilizing this orthorhombic-tetragonal morphotropic phase boundary.
A gray portion in FIG. 1 shows a principal composition range of a conventional type niobium based lead-free piezoelectric ceramic utilizing this orthorhombic-tetragonal morphotropic phase boundary.
On the other hand, according to Non-Patent Document 10, it was clarified that in lead based materials [(1−x)Pb(Zn1/3Nb2/3)O3-xPbTiO3], the piezoelectric characteristic is optimized by a sample having a composition neighboring to a rhombohedral-tetragonal morphotropic phase boundary, which is caused due to the fact that electric polarization rotates toward the (001) direction of a tetragonal crystal from the (111) direction of a rhombohedral crystal through the orthorhombic crystal or tetragonal crystal by an externally applied electric field.
Assuming that the same polarization rotation also contributes to a orthorhombic-tetragonal morphotropic phase boundary of the lead-free niobium based material (Na,K)NbO3, its change amount is merely partial as compared with a rhombohedral-orthorhombic-tetragonal change in lead based materials.
In consequence, it was considered that in the niobium based material, so far as this orthorhombic-tetragonal morphotropic phase boundary is utilized, the development of a lead-free piezoelectric ceramic having a piezoelectric characteristic comparable to lead based materials is accompanied with difficulty.    Patent Document 1: JP-A-2006206429    Patent Document 2: Japanese Patent Application No. 2005-502701    Patent Document 3: Japanese Patent Application No. 2005-513469    Patent Document 4: JP-A-2004-300012    Patent Document 5: Japanese Patent No. 3654408    Patent Document 6: Japanese Patent No. 3282576    Patent Document 7: Japanese Patent No. 3259677    Patent Document 8: Japanese Patent No. 3259678    Non-Patent Document 1: Jpn. J. Appl. Phys., 41, 7119 (2002)    Non-Patent Document 2: Ferroelectrics, 286, 93 (2003)    Non-Patent Document 3: Appl. Phys. Lett, 85, 4121 (2004)    Non-Patent Document 4: Jpn. J. Appl. Phys. Part 1, 43, 6662 (2004)    Non-Patent Document 5: Nature, 432, 84 (2004)    Non-Patent Document 6: Solid State Commun., 129, 274 (2004)    Non-Patent Document 7: Mat. Letts., 59, 241 (2005)    Non-Patent Document 8: phys. stat. sol. (a), 202, R57 (2005)    Non-Patent Document 9: Appl. Phys. Lett., 90, 092904 (2007)    Non-Patent Document 10: Nature, 403, 281 (2000)